Device and method for generating a local by micro-structure electrode dis-charges with microwaves

ABSTRACT

Device for producing a plasma, in particular for treating surfaces, for chemically reacting gases, or for producing light, by way of microstructure electrode discharges, using a device for producing plasma having at least one guide structure. A microwave generator which can be used to launch microwaves into the guide structure. The guide structure has a locally narrowly limited plasma region in contact with a gas. The guide structure is preferably a metallic waveguide filled with a dielectric material, or an arrangement of strip lines which run on a dielectric plate. The device and the method are particularly suited for processing or activating surfaces or for depositing layers on a substrate.

FIELD OF THE INVENTION

The present invention relates to a device and a method for producing aplasma, in particular for treating surfaces, for chemically reactinggases, or for producing light, by making use of microstructure electrodedischarges.

BACKGROUND INFORMATION

When treating surfaces using a plasma method, it is advantageous for theplasma to be produced as closely as possible to the surface or substrateto be treated, or for a plasma source having a sharply defined or localplasma volume to be introduced in close proximity to the substrate to betreated. This may be achieved by using so-called microstructureelectrode discharges, provision being made for dielectric plates havingelectrodes that are typically disposed at a distance of approximately100 μm or less from one another. As is generally known, discharges ofthis kind work within a very broad pressure range and exhibit relativelysharply delimited plasma interfaces, i.e., large-area, but locallynarrowly limited, small-volume plasmas are produced.

Microstructure electrode discharges have been ignited and operated byd.c. voltage. In this regard, reference is made, for example, to M. Rothet al., “Micro-Structure Electrodes as Electronic Interface BetweenSolid and Gas Phase: Electrically Steerable Catalysts for ChemicalReaction in the Gas Phase”, 1997, 1st Int. Conf. on MicroreactionTechnology, Frankfurt/Main and J. W. Frame, “Microdischarge DevicesFabricated in Silicon”, 1997, Appl. Phys. Lett., 71, 9, 1165.High-frequency or microwave excitations have not been implemented underknown methods heretofore.

It is also known from Kummer, “Grundlagender Mikrowellentechnik”(Fundamentals of Microwave Technology), VEB Publishers-Technology,Berlin, 1986, to direct microwaves via waveguides or strip waveguides(microstrip technology). In the case of the strip waveguides(microstrips), a metallic printed conductor, into which microwaves arelaunched, is usually applied to a dielectric substrate having a multiplygrounded metallic base plate. In the case that there is more than oneprinted conductor running on the base plate, the metallic base plate canbe eliminated.

SUMMARY OF THE INVENTION

It is believed that the device in accordance with the present inventionand its associated method have the advantage over the related art ofeliminating the need for the produced plasma to come into direct contactwith the device producing the plasma, and, in particular, with the partsof this device being used as electrodes. This may substantially prolongthe service life of the entire device in accordance with the presentinvention and, in particular, of the guide structure being used asmicrostructure electrodes. Moreover, the device in accordance with thepresent invention may be easier to service.

Moreover, due to the slight penetration depth of currents at highfrequencies, the electrode material (i.e., the guide structure (metallicwaveguide or strip waveguide) for guiding the launched microwaves in thedevice producing the plasma) can be kept very thin, which shouldsimplify fabrication. Thus, at a frequency of 2.45 GHz, depending on thematerial used, the requisite thickness may be merely a few μm. Thisapplies as well for structures or components used for launching themicrowaves into the guide structure. In particular, the guide structurecan be advantageously vapor-deposited, as well.

A locally or spatially narrowly bounded plasma is produced by microwavesin one or preferably in a multiplicity of plasma regions that areisolated from one another, by a supplied gas, which is directed past orthrough the guide structure, or which acts upon the guide structure.Thus, a gas plasma is produced at the surface of the guide structure, atleast on a region by region basis, in the plasma regions and in a plasmavolume defined by these regions.

Thus, it is quite beneficial for the service life of the device (i.e.,of the guide structure functioning as microstructure electrodes) for itto be coated with a protective dielectric layer in the vicinity of theplasma regions. Primarily suited for this are ceramic protective layers.The service life of the microstructure electrodes may be significantlyprolonged by this protective layer which cannot be used in a directvoltage operation.

Moreover, to fabricate the device, one can revert to existingtechnologies for generating plasma and, in particular, for guiding anddischarging the launched microwaves in the guide structure. Thus, themicrowaves are guided very advantageously via a known waveguide hollowconductor arrangement or a known micro-strip arrangement, which isproduced and structurally configured using likewise generally knownmicrostructuring methods.

The microwaves generated by a microwave generator are advantageouslylaunched into the guide structure via at least one launching structurewhich communicates electroconductively with the guide structure. Thefrequency of the supplied microwaves amounts advantageously to 300 MHzto 300 GHz.

As part of the device for generating the gas discharge and,respectively, the plasma, the guide structure for the injectedmicrowaves is in an exemplary embodiment a metallic waveguide, which isfilled with a puncture-proof, rigid dielectric material, such as silicondioxide. However, in an alternative exemplary embodiment, the guidestructure can be constructed of an arrangement of at least two,preferably parallel spaced metal plates, whose interstitial space isfilled in with a dielectric material. Due to its simpler structuraldesign, as compared to closed waveguides, this configuration may offeradvantages from a standpoint of production engineering.

The waveguides, the metal layers of the waveguides, or the metal plates,advantageously have a thickness, respectively a spacing, thatcorresponds to the penetration depth of the injected microwaves. Typicalvalues, known, for example from Kummer, “GrundlagenderMikrowellentechnik” Fundamentals of Microwave Technology), VEB TechnicalPublishers, Berlin, 1986, are within the μm range, given a typicalexpansion in the length and/or width of the waveguides, i.e., of themetal plates, in the cm range.

A particular benefit is derived when the H₁₀ mode of the launchedmicrowaves is excited and guided in the waveguide, as a guide structure,since, in this case, it is merely the width of the waveguide that iscritical for the propagation of the microwaves, and its length, forexample, apart from unavoidable attenuation, can be varied substantiallyfreely.

Alternatively, the guide structure can advantageously also be anarrangement made up of at least two metallic, in particular parallelconductive strips, which run on a dielectric plate. Here, as well,silicon dioxide is suited, for example, as material for the plate. Theseconductive strip lines are fabricated with a thickness of a fewpenetration depths, preferably using known microstructuring methods ormicrostrip structuring techniques.

In addition, provision is made in the vicinity of the guide structurefor at least one, but preferably for a multiplicity of, plasma regions,which are advantageously produced by a microstructuring of the guidestructure.

It is quite beneficial for these plasma regions to be cylindrical holesin the guide structure. Typical cylindrical hole diameters areadvantageously about 50 μm to 1000 μm. They are expediently distributedin a regular arrangement in the vicinity of the guide structure. In thecase of a waveguide as a guide structure, these cylindrical holes havethe considerable advantage, in combination with the excited H₁₀ mode,that the generated electrical field is aligned within the waveguide inparallel to the cylindrical holes and is substantially homogeneous. As aresult, variations in field strength in the direction of the waveguidewidth are minimal in comparison to higher excitable modes.

To avoid or minimize surface stress or material ablation andaccompanying gradual destruction of the plasma regions (i.e., of theguide structure) by the generated plasma, the inner wall of thecylindrical holes and, optionally, the entire electrode surfaces aswell, are advantageously provided with a dielectric, in particular aceramic protective layer. This dielectric protective layer onlymarginally degrades the propagation of the microwaves in the guidestructure.

The plasma is advantageously produced in the plasma-generation regionsat a pressure of 0.01 mbar to 1 bar, a microwave power of approximately1 mW to 1 watt being advantageously supplied to the plasma regions viathe microwave generator and the launching structure.

The supplied gas is preferably an inert gas, in particular argon, He orXe, as well as air, nitrogen, hydrogen, acetylene or methane, that ispreferably supplied with a gas flow of about 10 scam to about 1000 scam(standard cubic centimeters per minute). However, in the individualcase, these parameters are scaled by the selected dimensional size ofthe device for producing plasma and are merely to be considered astypical values. Another significant benefit is that the device inaccordance with the present invention can be operated while exposed toair, thereby achieving an oxidic surface excitation. Moreover, the broadpressure range within which the work can be done, from atmosphericpressure down to a precision vacuum, makes possible many diverseapplications.

The device in accordance with the present invention and the methodimplemented therewith are especially suited for processing or activatingthe surfaces of a substrate or for depositing layers. Its specialadvantage lies, in this context, in the spatially narrowly limitedextent of the plasma regions and in their immediate vicinity to thesubstrate surface to be treated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a device including a guide structure having cylindricalholes.

FIG. 2 depicts an alternative specific embodiment of the guidestructure.

FIG. 3 depicts a first gas guideway in the case of a plasma processingof a substrate using a guide structure.

FIG. 4 depicts an alternative specific embodiment including another gasguideway.

DETAILED DESCRIPTION

FIG. 1 illustrates a device 1 having a launching structure 10, a guidestructure 11, and plama regions 12. In this case, launching structure 10has the shape of a horn 20, as is generally known from microwavetechnology, and is used for launching microwaves into guide structure11. The microwaves are generated by a generally known microwavegenerator (not shown) which is linked to launching structure 10. Horn 20passes electroconductively over into guide structure 11, enablingmicrowaves to be launched by microwave generator via launching structure10 into guide structure 11.

In this example, guide structure 11 is designed as waveguide 21 of ametal, such as copper, high-grade steel, gold or silver, which is filledon the inside, for example, with silicon dioxide as rigid,puncture-proof, low-loss dielectric material 22. Waveguide 21 has athickness of up to a few mm. Its length is variable, but should amountto one fourth of the wavelength of the injected microwaves. Its width isdetermined in accordance with the waveguide mode selected.

In addition, waveguide 21 is provided with a multiplicity of cylindricalholes 26, which are configured in a regular arrangement and which defineplasma regions 12 located in the vicinity of cylindrical hole 26. Thediameter of individual cylindrical hole 26 amounts to about 50 μm to 1mm. Thus, device 1 is a microstructure, a plasma being ignited withineach plasma region 12 of guide structure 11 subsequent to the supplyingof a gas. Inner wall 23 of cylindrical holes 26 and, optionally, theentire electrode surfaces of guide structure 11 are also provided with adielectric, in particular a ceramic, coating as a protective layer,which is made, for example, of aluminum oxide or silicon dioxide.

The frequency of the microwaves launched into guide structure 11 isexpediently between 300 MHz to 30 GHz; preferably between 900 MHz and2.45 GHz are used. In this context, waveguide 21 is preferablydimensionally sized, and the frquency of the microwaves is preferablyselected such that the H₁₀ mode of the launched microwaves is excited inwaveguide 21 and propagates.

For this, in the individual case, one skilled in the art must match thewidth of waveguide 21 and the frequency of the microwaves to oneanother. For excitation of the H₁₀ mode, merely the width of waveguide21 is a critical quantity, while it length, for example, is merelyrelevant to the attenuation of the propagating microwave. The power ofthe launched microwaves is additionally selected to yield a power ofabout 1 mW to about 1 watt for each plasma discharge region 12.

FIGS. 3 and 4 elucidate the operation of device 1 for treating thesurface of a substrate 30 with a plasma through the microstructureelectrode discharges produced using device 1 in plasma regions 12 ofguide structure 11. To this end, in accordance with FIG. 3, a gas isdirected via a gas supply line 31 from the side facing away fromsubstrate 30 through cylindrical holes 26 of guide structure 11. Thus,this gas flows past the surface of substrate 30 and then off to theside. As of a minimal injected microwave power, which is essentially afunction of the type of supplied gas, the gas flow, the pressure, andthe thickness of waveguide 21, plasma is then generated in plasmaregions 12 essentially defined by the dimensions of cylindrical hole 26.Thus, located between guide structure 11 and substrate 30, at least on aregion by region basis, is a plasma volume 40, formed by various plasmaregions 12, which are isolated from one another or which merge,depending on the spacing between cylindrical holes 26.

The supplied gas is, for example, an inert gas, respectively a nobleinert gas, such as nitrogen or argon, for cleaning or activating thesurfaces of substrate 30. However, in the same way, it can also be agenerally known reactive gas, such as oxygen, air, acetylene, hydrogen,or a gaseous or vaporous precursor material, such as an organic siliconor organic titanium compound. Depending on the selection of the suppliedgas, chemical reactions can also be induced by device 1 at the surfaceof the substrate, or a surface coating can be provided, for example inthe form of a hard material coating or wear-protection layer.

The plasma is produced in plasma region 12 with the aid of microwaveslaunched into guide structure 11 and with the supplying of a gas, anddepends on the dimensional design of guide structure 11, the type ofsupplied gas, the diameter of cylindrical holes 26, the width ofwaveguide 21, and the desired treatment of the surface at a pressure ofabout 0.01 mbar up to about 1 bar. Each variable is to be determined inthe individual case by one skilled in the art based on simplepreliminary tests. A preferred pressure is from 10 mbar up to 200 mbar,with plasma gas being supplied with a typical gas flow of a few sccm upto about 1000 sccm. However, this value is likewise to be adapted by oneskilled in the art to the particular process parameters for each case,after performing preliminary tests.

As a second exemplary embodiment, FIG. 4 depicts an alternative routingof the supplied gas via gas supply line 31. In this context, the gasflows past, in between the surface of substrate 30 and guide structure11, and is not fed through cylindrical holes 26. Apart from that,however, the parameters for producing the plasma in plasma regions 12are completely analogous to the exemplary embodiment elucidated with theaid of FIGS. 1 and 3.

In a third exemplary embodiment, as a slight variation of waveguide 21,guide structure 11 is made of two parallel spaced metal plates, whoseinterstitial space is filled with silicon dioxide. Apart from that,guide structure 21 is constructed substantially similar to the firstexamplary embodiment and FIG. 1, especially with respect to dimensionaldesign, cylindrical holes, and material. The advantage of using twoparallel metal plates in place of waveguide 21 is that, from astandpoint of production engineering, they are simpler and lessexpensive to fabricate than a closed, integrated, waveguide 21. In thiscase, the guidance and propagation of the launched microwaves is carriedout by way of a capacitive coupling of the two plates. Analogously tothe preceding exemplary embodiments, the gas is supplied in thisexemplary embodiment in the manner explained with respect to FIG. 3 or4.

As a further exemplary embodiment, FIG. 2 clarifies an alternativespecific embodiment of guide structure 11, the launched microwaves beingguided via strip lines 24 using microstrip technology. In this case,horn 20 is not necessary since the microwaves generated by the microwavegenerator are injected via coaxial plug connectors (not shown).

In detail, in this example at least two, but preferably a multiplicityof, metallic strip lines 24 are applied to a dielectric plate 25, whichis made of a puncture-proof, rigid dielectric material, such as silicondioxide. These strip lines 24 expediently run in parallel to one anotherat a distance that is a function of the frequency and the dielectricmaterial used, and are preferably made of copper or gold, which isoptionally applied to a galvanic reinforcement, such as nickel. Theoptimal spacing of strip lines 24 for igniting and sustaining a plasmain plasma regions 12 is additionally a function of the type of gassupplied and of the prevailing pressure and must, therefore, bedetermined in simple preliminary tests.

Furthermore, analogously to FIG. 1, cylindrical holes 26 are provided indielectric plate 25 between strip lines 24. With respect to thedimensional design of guide structure 11 and of cylindrical holes 26,reference is made to the preceding explanations regarding the firstexemplary embodiment. In particular, in this case as well, cylindricalbores 26 can be provided with a dielectric coating, for example in theform of a ceramic protective layer, on inner wall 23. Cylindrical bores26, in turn, define locally limited plasma regions 12, in whichmicrostructure electrode discharges are ignited via the injectedmicrowaves directed via strip lines 24 in response to the supplying of agas or on exposure to air. When cylindrical holes 26 are arranged in adense enough configuration, the plasmas produced in plasma regions 12merge, and a laterally homogeneous plasma develops.

In the case of a guide structure 11 in accordance with FIG. 2, the gasguidance is completely analogous to the exemplary embodiments alreadyexplained and can be carried out in the manner explained with respect toFIG. 3 or 4, in that the gas is directed through cylindrical holes 26 orconveyed between substrate 30 and guide structure 11.

REFERENCE SYMBOL LIST

-   1 device-   10 launching structure-   11 guide structure-   12 plasma region-   20 horn-   21 waveguide-   22 dielectric material-   23 inner wall-   24 strip line-   25 dielectric plate-   26 cylindrical hole-   30 substrate-   31 gas supply line-   40 plasma volumes

1. A device for producing a plasma through microstructure electrodedischarges, a use of the plasma including at least one of treatingsurfaces, chemically reacting gases, and producing light, the devicecomprising: at least one guide structure, the at least one guidestructure including at least one hole, wherein a plasma region includesat least one of the hole and an area adjacent to the hole; a microwavegenerator, the microwave generator launching electromagnetic microwavesinto the at least one guide structure to produce the plasma, the plasmabeing produced in the plasma region, wherein the at least one guidestructure is a metallic waveguide filled with a dielectric material, thedielectric including at least one of silicon dioxide, ceramic, andKapton; and further comprising: an arrangement of at least two spacedmetal plates, the at least two spaced metal plates forming aninterstitial space filled with a dielectric material.
 2. The device ofclaim 1, wherein the metallic waveguide has a thickness.
 3. The deviceof claim 1, wherein an H₁₀ mode of the microwaves is launched into theat least one guide structure.
 4. A device for producing a plasma throughmicrostructure electrode discharges, a use of the plasma including atleast one of treating surfaces, chemically reacting gases, and producinglight, the device comprising: at least one guide structure, the at leastone guide structure including at least one hole, wherein a plasma regionincludes at least one of the hole and an area adjacent to the hole; anda microwave generator, the microwave generator launching electromagneticmicrowaves into the at least one guide structure to produce the plasma,the plasma being produced in the plasma region, wherein the at least oneguide structure is an arrangement of at least two spaced metal plates,the at least two spaced metal plates forming an interstitial spacefilled with a dielectric material.
 5. The device of claim 4, wherein themetal plates have a spacing of 10 mm to 1000 mm.
 6. A device forproducing a plasma through microstructure electrode discharges, a use ofthe plasma including at least one of treating surfaces, chemicallyreacting gases, and producing light, the device comprising: at least oneguide structure, the at least one guide structure including at least onehole, wherein a plasma region includes at least one of the hole and anarea adjacent to the hole; and a microwave generator, the microwavegenerator launching electromagnetic microwaves into the at least oneguide structure to produce the plasma, the plasma being produced in theplasma region, wherein the at least one guide structure is anarrangement of at least two metallic strip lines, the at least twometallic strip lines running on a dielectric plate.
 7. A device forproducing a plasma through microstructure electrode discharges, a use ofthe plasma including at least one of treating surfaces, chemicallyreacting gases, and producing light, the device comprising: at least oneguide structure, the at least one guide structure including at least onehole, wherein a plasma region includes at least one of the hole and anarea adjacent to the hole; and a microwave generator, the microwavegenerator launching electromagnetic microwaves into the at least oneguide structure to produce the plasma, the plasma being produced in theplasma region, wherein the at least one guide structure is at least oneof planar, curved, cylindrical and coaxial, the at least one guidestructure including an internal, central conductor.